package uk.co.q3c.deplan.client.domain.dag;

/*
 * Copyright The Codehaus Foundation.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may
 * obtain a copy of the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions
 * and limitations under the License.
 */

import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;

/**
 * @author <a href="michal.maczka@dimatics.com">Michal Maczka</a>
 * @version $Id: CycleDetector.java 8010 2009-01-07 12:59:50Z vsiveton $
 */
public class CycleDetector<T> {

	private final Integer NOT_VISTITED = new Integer(0);

	private final Integer VISITING = new Integer(1);

	private final Integer VISITED = new Integer(2);

	public List<T> hasCycle(final DAG<T> graph) {
		final List<Vertex<T>> verticies = graph.getVerticies();

		final Map<Vertex<T>, Integer> vertexStateMap = new HashMap<Vertex<T>, Integer>();

		List<T> retValue = null;

		for (final Iterator<Vertex<T>> iter = verticies.iterator(); iter.hasNext();) {
			final Vertex<T> vertex = iter.next();

			if (isNotVisited(vertex, vertexStateMap)) {
				retValue = introducesCycle(vertex, vertexStateMap);

				if (retValue != null) {
					break;
				}
			}
		}

		return retValue;

	}

	/**
	 * This method will be called when an edge leading to given vertex was added and we want to check if introduction of this edge has not
	 * resulted in a cycle in the graph
	 * 
	 * @param vertex
	 * @param vertexStateMap
	 * @return
	 */
	public List<T> introducesCycle(final Vertex<T> vertex, final Map<Vertex<T>, Integer> vertexStateMap) {
		final LinkedList<T> cycleStack = new LinkedList<T>();

		final boolean hasCycle = dfsVisit(vertex, cycleStack, vertexStateMap);

		if (hasCycle) {
			// we have a situation like: [b, a, c, d, b, f, g, h].
			// Label of Vertex which introduced the cycle is at the first position in the list
			// We have to find second occurence of this label and use its position in the list
			// for getting the sublist of vertex labels of cycle paricipants
			//
			// So in our case we are seraching for [b, a, c, d, b]
			final T label = cycleStack.getFirst();

			final int pos = cycleStack.lastIndexOf(label);

			final List<T> cycle = cycleStack.subList(0, pos + 1);

			Collections.reverse(cycle);

			return cycle;
		}

		return null;
	}

	public List<T> introducesCycle(final Vertex<T> vertex) {

		final Map<Vertex<T>, Integer> vertexStateMap = new HashMap<Vertex<T>, Integer>();

		return introducesCycle(vertex, vertexStateMap);

	}

	/**
	 * @param vertex
	 * @param vertexStateMap
	 * @return
	 */
	private boolean isNotVisited(final Vertex<T> vertex, final Map<Vertex<T>, Integer> vertexStateMap) {
		if (!vertexStateMap.containsKey(vertex)) {
			return true;
		}

		final Integer state = vertexStateMap.get(vertex);

		return NOT_VISTITED.equals(state);
	}

	/**
	 * @param vertex
	 * @param vertexStateMap
	 * @return
	 */
	private boolean isVisiting(final Vertex<T> vertex, final Map<Vertex<T>, Integer> vertexStateMap) {
		final Integer state = vertexStateMap.get(vertex);

		return VISITING.equals(state);
	}

	private boolean dfsVisit(final Vertex<T> vertex, final LinkedList<T> cycle, final Map<Vertex<T>, Integer> vertexStateMap) {
		cycle.addFirst(vertex.getKey());

		vertexStateMap.put(vertex, VISITING);

		final List<Vertex<T>> verticies = vertex.getChildren();

		for (final Iterator<Vertex<T>> iter = verticies.iterator(); iter.hasNext();) {
			final Vertex<T> v = iter.next();

			if (isNotVisited(v, vertexStateMap)) {
				final boolean hasCycle = dfsVisit(v, cycle, vertexStateMap);

				if (hasCycle) {
					return true;
				}
			} else if (isVisiting(v, vertexStateMap)) {
				cycle.addFirst(v.getKey());

				return true;
			}
		}
		vertexStateMap.put(vertex, VISITED);

		cycle.removeFirst();

		return false;

	}

}